1. Field of the Invention
The invention relates to an apparatus for identifying sheet-like printed matters and, more particularly, to one which identifies whether a sheet-like material is true or false.
2. Description of the Prior Art
Already known is an apparatus which identified objects e.e. passenger ticket and securities as true ones or false ones, and which is used with a transporting means such a belt. The phrase "true and false" is used here in two senses. First, it means that an object is fit or unfit. More specifically, even though a bank note is genuine, for example, it is considered unfit for recirculation if it is stained too much, torn too much or has an adhesive tape on it. Secondly, it means that an object such as note and securities is genuine or counterfeit.
A conventional identifying apparatus of this type is provided with three opto-electric converting elements such as photo diodes disposed at both sides of a transporting path of the object. Those three opto-electric converting elements sense red, green and blue color components included in incident rays of light, respectively. When there is no object in an object information detecting area on the object transporting path, those converting elements directly receive reference rays of light emitted from a reference light source. On the other hand, when the object exists in the area, the elements receive the reference rays transmitted through or reflecting from the object. The outputs of the converting elements are amplified by the corresponding amplifying circuits to have proper amplitudes, respectively. The amplifying circuits have automatic gain control circuits associated therewith. The output signals from the amplifier circuits are integrated for a given time by integration circuits provided corresponding to the amplifier circuits under control of timing signals delivered from a system control circuit. The integrated data from the integration circuits are applied to a division circuit, an adder circuit and the like where those are properly processed, and then are applied to corresponding comparators. In the comparators, true object data read out from a true object information memory previously storing true object information are compared with the operated data of the integrated ones for judging whether the object is true or false.
The automatic gain control circuits respectively control the gains of the corresponding amplifying circuits so that the output signals V1, V2 and V3 from the amplifying circuits are so related as to be V1=V2=V3 during a period that the opto-electric converting elements receive the reference ray, under control of the control signals from the system control circuit. During a time period that the opto-electric converting elements are receiving light rays transmitted through or reflecting from the object, each feedback loop for the gain control is open.
The following problems are involved in the conventional identifying apparatus, however. Although a high precision is required for the gain control of each automatic gain control circuit, it is in fact difficult to well balance the mutual adjustments among the gain control circuits. This results in degradation of the identifying or judging accuracy of the apparatus. Further, it is impossible to remove detrimental and varying factors inherently included in the gain control circuits. Moreover, since the integrating time of the output signals from the amplifying circuits are prefixed, a variation of the speed of the moving transport belt directly appears as an error of the detected signal. In other words, it is impossible to remove an error of the detected signal arising from a variation of the transporting speed of the belt.